It has become conventional to impart, specific functions, such as ornamentation or functional indication, to a molding prepared from a thermoplastic resin by forming indicia or the like on its surface.
Exemplary applications where functional indications are desired include uses where there is a high frequency of contact, such as the keys for personal computers, word processors, typewriters, desktop electronic calculators or telephone sets, or various switches and control levers of an automobile. These end use applications, especially require that the indicia characters or the like formed on the moldings surface exhibit wear-resistance, otherwise the indicia will fade due to repeated contact during use.
Since thermoplastic resins, especially, engineering plastics and crystalline thermoplastic resins are generally chemically stable (and thereby insufficiently reactive) and form moldings having hard and smooth surfaces, conventional printing processes, such as forming indicia or the like on the resin molding by screen printing, pad printing, or offset printing and then fixing the ink by drying at normal and at normal pressure, usually will not promote strong adhesion of the ink to the resin. Thus, conventional processes have been unable to meet the requirements described above even though they are satisfactory for end use applications where low durability is required. Furthermore, conventional processes are inefficient since a long time, usually 20 to 30 minutes, so as to fix the ink by drying at normal or elevated temperature.
Broadly, the present invention is directed to fixing indicia or the like on a resin molding by blowing a high-temperature compressed gas against the printed area as an aftertreatment.
More specifically, the present invention relates to a process for printing, characters or the like on a thermoplastic resin molding by forming them on the molding by printing, and then blowing a high-temperature compressed gas against the printed surface of the molding so that the temperature and pressure of the compressed gas is between 100.degree. to 300.degree. C. and 0.05 to 5 kg/cm.sup.2, respectively, or the like so as to fix the indicia like thereto.